The invention relates to devices for dispensing, delivering, administering or injecting substances and to methods for making and using them. More particularly, it relates to a medical device for the administration, including metered administration, of a liquid product or substance that contains a medical or therapeutic agent, more particularly insulin for treating patients with diabetes.
For metered administration, liquids that contain a medical or therapeutic agent are commonly stored in a liquid receptacle in which a piston plug is mounted so that it is axially movable. The piston plug is moved axially toward an outlet of the liquid receptacle by an output member of a drive mechanism, so as to cause liquid to be dispensed from the liquid receptacle. It is preferable in this case that the liquid receptacle is exchangeable. This requires a connecting mechanism for releasably connecting the piston plug to the output member. For exact metering of the liquid that is to be administered, the connecting mechanism must be such that the connection between piston plug and output member is secured against axial displacement of the output member relative to the piston plug, for example in order to prevent the liquid receptacle from self-emptying as a result of what is known as a siphoning effect.
From the prior art, it is known to provide the output member with a bead which engages in an undercut on the rear face of the piston plug. Such a connection can be established and released again only with difficulty. From the prior art, it is also known to provide the piston plug and the front end of the output member with a thread and an opposing thread, respectively, so that a connection can be established by screwing the output member onto the piston plug and this connection can be released again by unscrewing the output member from the piston plug. The ampoule, with the piston plug held in it, is usually screwed onto the output member by hand. This method of connecting may be difficult for some people, for example the elderly, frail or those with joint problems. It is time-consuming, and problematic if the positions of the output member and of the piston plug do not agree or if they are not aligned properly. During screwing, liquid may be inadvertantly dispensed or air may be drawn into the liquid receptacle. Intake of air should be avoided in particular, as this can lead to therapy complications.